[refactor] Sync with master

This was initially a merge commit that got ironed out by rebase cleanup.
Notable changes are:
- Copying EntityRef instead of referencing it
- Some changes to includes

src/Groups.hpp

@@ -1,13 +1,8 @@
 #pragma once
 
-#include "Entities.hpp"
+#include "EntityRef.hpp"
 
 #include <vector>
-#include <map>
-#include <list>
-
-/* forward declaration(s) */
-struct Player;
 
 struct Group {
     std::vector<EntityRef> members;